Polymeric materials are used as catheters in many clinical situations every day. Typical materials such as medical grade silicone elicit a lower immunogenic response than previously used materials (polyurethanes or polyethylenes). Even with this development, silicone catheters have significant complications during acute and chronic use. These include biofilm formation, encrustation and bacterial infection.
In 2009, roughly 40% of infections acquired in U.S. hospitals were caused by urinary catheters, costing the healthcare system over $1 billion. Central venous catheters are estimated to cause roughly 80,000 bloodstream infections, resulting in 28,000 deaths and costing up to $2.3 billion annually. The most common pathogens to induce an infection include gram-positive staphylococcus aureus and staphylococcus epidermidis, as well as gram-negative Escherichia coli and pseudomonas aeruginosa. Bacteria can come from a variety of sources, including the operating room atmosphere, surgical equipment, clothing from medical staff, and even bacteria on the patient's skin and already in their body. Therefore, even under sterilized conditions, it can be difficult to prevent infections. In addition to infections, long term urinary catheterization increases the likelihood of encrustation, which is the buildup of minerals on the catheter and can potentially lead to urinary blockage.
The mechanism for bacterial adhesion onto a catheter occurs via a three step process: 1-2 hours after implantation, non-specific and reversible bonding occurs through gravitational, van der Waals, electrostatic, hydrogen bond, dipole-dipole, ionic bond, and hydrophobic interactions; roughly 2-3 hours later, stronger adhesion occurs via specific chemical interactions between the bacteria and substrate surface, forming irreversible bonds; and finally, if sufficient nutrients are supplied, a biofilm can form on the implant surface. Once a biofilm forms, roughly 1,000 times the antibiotic dose is required to treat the infection compared to killing the bacteria in suspension. In addition to infections, long term catheterization increases the likelihood of encrustation, which is the buildup of minerals on the catheter and can potentially lead to urinary blockage. For these reasons, there is a high interest in developing catheter materials that prevent bacterial and mineral adhesion.
The Foley catheter, which is the most commonly used device for urinary catheterization, is often made of silicone materials due to its low immunological response. A lubricious coating is often added in order to reduce urethral irritation. To address the issues of infections, some catheters are being coated with antimicrobial agents such as silver alloys or nitrofurazone. While antimicrobial coatings may cost up to twice as much as an uncoated catheter, their higher costs are becoming justified due to their ability to reduce infection rates, which can cost between $3,700 and $56,000 per patient. Despite the antimicrobial coatings' ability to reduce medical costs and infection rates compared to uncoated catheters, they are still not completely effective against preventing infections. One drawback is that most antimicrobial agents are not effective against all pathogens, especially antibiotic-resistant bacteria. In addition, antimicrobial coatings are not always effective against preventing biofilm formation for long-term catheterization. Even though antimicrobial coatings may kill the initial bacteria they interact with, the dead bacteria will still stick to the catheter, providing a perfect layer for new bacterial attachment and biofilm formation. Finally, in areas where the coating has been delaminated and the underlying catheter is exposed, bacteria can adhere very quickly since current catheter materials do not have an innate ability to prevent bacterial adhesion. For all these reasons, the likelihood of a catheter induced infection is directly correlated to the duration of catheterization, with the chances of infection increasing 3-10% daily.
To mitigate these problems, many patients have turned to intermittent catheters, which are disposable catheters and are used each time the bladder is emptied. Revenues for intermittent catheters were $143 million in 2009. While the chance for infection is greatly reduced, patients must be highly trained to use intermittent catheters since they can potentially cause urethra problems due to irritation. They are also expensive since they need to be replaced often.